1. Field of the Invention
The present invention relates to cathode ray tube (CRT) display apparatus for displaying successive frames of a non-interlaced video image with improved character legibility.
2. Description of the Related Art
A CRT display, such as a computer visual display unit or a television receiver for example, conventionally comprises a line timebase circuit for sequentially addressing one or more electron beams to successive pixels in a line from one side of a CRT to the other in response to a line timebase synchronization signal, and a frame timebase circuit for sequentially addressing each electron beam to successive lines from the top of the CRT to the bottom in response to a frame timebase synchronization signal. In operation, each electron beam follows a zigzag pattern over the area of the CRT screen. The zigzag pattern is usually, and will hereinafter be, referred to as a raster.
In the interests of maximizing the legibility of the video image, it is desirable to minimize the spacing between adjacent lines of the raster. This requires a high raster line density and a small electron beam spot size. However, in a color CRT, if the spot size is sufficiently small, the temporal frequency at which the electron beams are scanned across the lines of the raster can interact with the spatial frequency of the perforations in the shadow mask to generate an objectionable Moire fringe pattern on the CRT screen. Moire fringe patterns are especially common in displays capable of operating at any one of a range of line scan frequencies because it is difficult to optimize the spatial frequency of the perforations in the shadow mask to prevent them at all frequencies in the range. This problem can be overcome using a larger spot size. However, this militates against obtaining a high contrast ratio between elements of characters such as "m" and "e".